1. Field of the Invention
The present invention relates to an electric power steering control system for assisting steering force with a motor.
2. Description of the Prior Art
FIG. 16 is a block diagram showing the constitution of an electric power steering control system of the prior art disclosed in Mitsubishi Technical Report Vol. 70, No. 9, pp.43-48, for example. In FIG. 16, reference numeral 1 denotes a torque sensor for detecting steering torque when a driver steers an automobile; 2, a phase compensator for improving the frequency characteristics of the output signal of the torque sensor 1; 3, a torque controller for computing an assist torque current for assisting the steering torque based on the phase compensated output of the torque sensor 1; 4, a damping controller for computing a damping current based on the rotation angle speed .omega.M of a motor 8 computed by unshown motor angle speed computing means, for example, 5 a compensation controller which consists of a friction compensator 5a for computing a friction compensation current for compensating the friction torque of the motor 8 and an inertia compensator 5b for computing an inertia compensating current for compensating the inertia moment of the motor 8 and which computes the friction computing current based on .omega.M and inertia compensating current based on the rotation angle acceleration (d.omega.M/dt) obtained by differentiating .omega.M, and 6 an adder for computing a target current by adding the assist torque current computed by the torque controller 3, the damping current computed by the damping controller 4, and the friction compensating current and the inertia compensating current computed by the compensation controller 5. Denoted by 7 is a current controller for comparing the target current computed by the adder 6 with the drive current of the motor 8 output from a current detector 9 and carrying out current control to ensure that the drive current becomes equal to the target current.
A description is subsequently given of the operation of the electric power steering control system of the prior art.
When a driver operates the steering wheel of the automobile, steering torque at this point is measured by the torque sensor 1, phase compensated by the phase compensator 2 to improve the frequency characteristics thereof, and applied to the torque controller 3. The torque controller 3 computes an assist torque current almost proportional to the output signal of the torque sensor 1 whose frequency characteristics have been improved, the steering torque of the driver is assisted by driving the motor 8 based on the assist torque current, and the steering torque of the driver is thereby reduced.
At this point, to stabilize the movement of the steering wheel, the damping controller 4 computes a damping current proportional to the motor rotation angle speed .omega.M, and the damping current is added to the above assist torque current. To compensate for the influence of the friction of the motor 8, the friction compensation controller 5a adds a friction compensating current which changes according to the sign of the motor rotation angle speed .omega.M. To compensate for the influence of the inertia moment of the motor 8, the inertia compensation controller 5b adds an inertia compensating current proportional to the motor rotation angle acceleration (d.omega.M/dt) obtained by differentiating the motor rotation angle speed .omega.M. These compensating currents are added to the assist torque current to compute a target current, and the current controller 7 controls a drive current to be supplied to the motor 8 based on the target current to generate assist torque proportional to the drive current, thereby reducing the steering torque of the driver and stabilizing the movement of the steering wheel. The controllers 3, 4 and 5 change control parameters according to car speed.
The assist torque current computed by the torque controller 3 takes a value almost proportional to the output signal of the torque sensor 1 whose frequency characteristics have been improved by the phase compensator 2. At this point, the larger the torque proportional gain set by the torque controller 3 the greater the assist torque becomes, thereby making it possible to reduce the steering torque of the driver. However, when the torque proportional gain is increased, oscillation of a control system occurs and the driver feels uncomfortable torque oscillation. Therefore, the torque proportional gain cannot be simply increased.
To prevent the oscillation, means of increasing a damping current is conceivable. However, in the prior art, when compensation is made to increase the damping current, this damping compensation functions as resistance when the steering wheel is turned, thereby increasing steering torque. Therefore, a large damping current cannot be added. Consequently, the torque proportional gain cannot be increased and the steering torque of the driver cannot be fully reduced when great assist torque is required as in the case where the steering wheel is turned while an automobile is stopped.
It is an object of the present invention, which has been made to solve the above problems, to provide an electric power steering control system which can reduce steering torque without making a driver feel uncomfortable torque oscillation.